This makes two changes:
- Adds a way to register a callback for when a FlutterDesktopPluginRegistrarRef is destroyed, and implements the logic to call it in the Windows and Linux embeddings.
- Adds a class to the C++ wrapper that handles making a singleton owning PluginRegistrar wrappers, and destroying them when the underlying reference goes away, to avoid needing that boilerplate code in every plugin's source.

Fixes https://github.com/flutter/flutter/issues/53496

The JSON codec is awkward to use in the wrapper (since the client has to build and link one of the JSON libraries to do so). Since it would be very cumbersome to wrap in a C API, and there's essentially no reason to use it instead of the standard codec, this removes it from the wrapper entirely.

Since some system channels (internal to the engine) still use it, it's moved into common/cpp instead of being eliminated entirely. Internally we always use RapidJSON though, so the jsoncpp implementation is removed. Also adds some unit test coverage, since there wasn't any.

Fixes #30669

This is a step toward aligning the API with macOS, and will make it easier to add the precompiled library later for release mode (since it can just be added to the project directory, without any code changes required for wrapper clients).

At the C API, uses a struct instead of individual arguments, mirroring a change that was already made on the Linux side to make the C API cleaner.

Functional changes in addition to the restructuring:

adds relative path support, as was recently added for GLFW
Uses wstring, rather than string, for paths; the conversion to UTF-8 is actually a potential problem on Windows, so pushing it into the embedding allows us the possibility of removing it later (if we can figure out a good solution at the embedder.h layer) without API breakage.
The old APIs used by the standard runner are left in place for now to avoid breaking the template on an engine roll. Once the framework template has been updated, the old API paths will be removed.

Clang has different warning settings, so catches different issues than
the VS compile. This fixes various minor issues caught by clang.

Part of https://github.com/flutter/flutter/issues/16256

This was only necessary when the Engine had to build in multiple buildroots
where the sources where checked out at different paths relative to the
buildroot. This is no longer the case and there are already cases GN rules
have been written that mix and match variable usage with the direct
specification of the path to the Flutter sources relative to the sole buildroot.

plugin_registrar_windows.h was never fully updated for the Win32 switch,
and didn't actually compile. This introduces FlutterView (parallel to
the GLFW wrapper's FlutterWindow) as a ways of holding view-specific
functionality to expose via the plugin registrar, and moves HWND access
from the FlutterViewController to the FlutterView so that it's
available to plugins. This allows the implementation of plugins that need
access to the native HWND (e.g., moving or resizing the top-level window).

Adds simple unit tests of the new wrapper functionality, ensuring that the
files actually compile, and that the passthroughs work as expected.

This is a breaking change for Windows runners due to moving
GetNativeWindow() in the wrapper. It's not being done as a multi-stage
change (addition + deprecation + later removal) since this API is explicitly
unstable.

Moved our code to passing functions by const ref

Adds a task runner, and exposes API to allow application-level runloops to know when they need to next call the API to process engine events. Internally, sends null events to wake up the app runloop when new events are scheduled to ensure the wait time is updated accordingly.

Fixes #36420

* Add windows font change logic

* update

* fix comment

Makes the plugin registration structure consistent with macOS. This will
be used in generated plugin registrant files rather than a specific
implemenation class, so this helps unblock the creation of generated
registrants on Windows and Linux.

Significantly improves the behavior of non-ASCII text input on Windows. Correctly
processes incoming character events as UTF-16, and for now uses UTF-32 for
the text model so that the existing index-based logic will work much more often.

Future work is still needed, but this will handle far more cases correctly.

Add a context for async resource uploading.

Fixes flutter/flutter#36422

* Begin API evolution to a more native win32 API

* Child-window based hosting

* Plumb through an initial size for child window to avoid reallocated surface on start

* Windows API cleanup part 1

* Fix wrapper tests

* Ensure flutter's HWND resources are destroyed

* Final API cleanup

* Fix dynamic DPI handling

* Cleanup

* Fix a bug that was causing engine to not be shutdown correctly

* CR feedback

* auto format

* CR feedback: combine FlutterView and FlutterViewController

* The one that clang-format seems to always get wrong

* expletive

* fix sources for licesnse file

* CR Feedback

* cleanup

* Update GetNativeWindow() to return an HWND rather than a long

* fix formatting

We are switching to -Wnewline-eof soon and these will become errors. Anyway, this was undefined behavior.

Final portion of reworking the names of the Linux and Windows desktop build outputs.

See flutter/flutter#38589

Part of restructuring the artifacts to support transitioning away from
GLFW embeddings on desktop.

https://github.com/flutter/flutter/issues/38589

Start work on flutter/flutter#30726 by adding an alternative win32 shell platform implementation for Windows that is not based on GLFW and that uses LIBANGLE for rendering and native win32 windowing and input. This change does not replace the GLFW implementation but rather runs side by side with it producing a secondary flutter_windows_win32.dll artifact. The following items must be added to attain parity with the GLFW implementation:
- Custom task scheduling
- Support for keyboard modifier keys
- Async texture uploads
- Correct high DPI handling on Windows versions < 1703
and will be added in subsequent changes.

Enables the build of the Windows shell, based on the same GLFW code used
by the current Linux shell.